1. Field of the Invention:
This invention relates to a sealing cover which is particularly suitable for the hermetic sealing of a semiconductor device, and a method of producing the same.
2. Description of the Prior Art:
There is known a ceramic package for a semiconductor element as is typically shown in FIG. 2. It includes a ceramic substrate 1 which comprises an integral assembly of a lower sheet having at its center a metallized layer for bonding a semiconductor element, a middle sheet carrying a lead pattern and having an opening at its center and an upper sheet having an opening which is larger than the opening of the middle sheet to allow the exposure of the inner ends of the lead pattern therethrough. A plurality of metallic leads 2 are bonded to the longitudinal edges of the substrate for connection to the outer ends of the lead pattern. The edge of the upper sheet surrounding its opening has a metallized layer 3 to which a cover is attached. The metallized layer 3 and the lead pattern are usually formed from an electrically conductive paste of a compound of molybdenum and manganese. The leads 2, the metallized layer 3 and the lead pattern are usually plated with gold.
The package is usually produced by a process which comprises bonding the semiconductor element 4 to the central recess of the substrate 1, connecting the electrodes on the element 4 to the inner ends of the lead pattern by thin connector wires 5, placing a seal ring 6 and a metallic cover 7 on the metallized layer 3, heating the seal ring 6 to a temperature not lower than its melting point, cooling it and attaching the cover 7 to the seal ring. The step of attaching the cover 7 is usually called hermetic sealing. While a Au-Si alloy solder is used for bonding the semiconductor element 4, a solder having a lower melting point, such as a Au-Sn or Pb-Sn alloy solder, is used for the seal ring 6. The cover 7 is usually formed from Kovar (tradename of an Fe-Ni-Co alloy). At least those edge portions of the cover 7 which are brought into contact with the seal ring 6 are usually coated with a film of, say, gold or nickel having good solderability.
One of the important things in the step of hermetic sealing is to place the seal ring 6 and the cover 7 on the metallized layer 3 so that their peripheral edges may be aligned with each other substantially perfectly. This is, however, a very difficult job, since the seal ring 6 has a thickness of only about 50 microns which makes it very difficult to handle and liable to displacement by even a slight vibration. A hermetically sealed assembly in which the cover 7 is not correctly positioned is often produced. This problem can be overcome if the seal ring 6 can be attached to the cover 7 beforehand. Therefore, there are known various methods which employ the spot welding of the seal ring 6 to the cover 7 at a plurality of spots, as disclosed in, for example, U.S. Pat. Nos. 3,823,468, 3,874,549 and 3,946,190. The spot welding, however, gives rise to a number of problems. For example, the solder is melted at each welded spot and the molten solder and the metal coating on the cover 7 form a material of different composition having a higher melting point. As a natural consequence, the seal ring 6 fails to melt uniformly during the step of hermetic sealing. Another problem is due to the welding rod which is used for spot welding. It leaves impressions at the welded spots and they are likely to form voids during the step of hermetic sealing.